Many types of electrical equipment contain a mineral insulating oil for dissipating the heat generated by energized components, for insulating the energized components from the equipment enclosure and from other internal parts and devices, and combinations thereof. Examples of electrical equipment include, but are not limited to, transformers, capacitors, switches, regulators, circuit breakers, cables, reclosers, x-ray equipment, and combinations thereof.
A transformer generally transfers electric power from one circuit to another electromagnetically. Transformers are generally used in the transmission of electrical power. Large transformers generally require insulation of coils, conductors, and combinations thereof, in order to protect the transformer at normal operating voltages, during temperature overvoltages, during transient overvoltages, and combinations thereof. Transient overvoltages may result from lightning strikes, switching operations, and combinations thereof. When insulation fails, an internal fault or short circuit may occur. Such occurrences may cause the equipment to fail, typically leading to system outages and possibly endangering persons in the vicinity of the equipment.
In order to effectively transfer heat away from a transformer core and coil assembly and to maintain an acceptable operating temperature, conventional transformers use relatively large volumes of a mineral insulating oil as insulation.
In the past, mineral insulating oils made from naphthenic or paraffinic base oils tended to have inherently poor low temperature viscometric properties and generally did not exhibit low gassing performance as required by American Standard Test Method (ASTM) D3487 for Type I mineral insulating oils.
In addition, the gassing tendency of a mineral insulating oil is a measure of the rate of absorption or desorption of hydrogen into or out of the mineral insulating oil under prescribed laboratory conditions. Low gassing performance is important because, if hydrogen is evolved due to electrical stress, a liquid having low gassing tendency tends to absorb the evolved hydrogen and thereby reduce the chances of an explosion.
Naphthenic base oils and paraffinic base oils may be designed for use in mineral insulating oil applications. Naphthenic base oils may need to be chemically inhibited to control oxidation tendencies in meeting industrial requirements. Naphthenic base oils have good low temperature properties due to low wax concentrations. Whereas many paraffinic base oils are oxidatively stable, the paraffinic base oils have high positive gassing tendencies and poor low temperature performance (high pour point) in mineral insulating oil applications.
U.S. Pat. No. 6,355,850 B1 to Angelo et al. discloses that electrical oils having improved uninhibited oxidation and electrical resistance are derived by blending a substantially nitrogen and sulfur free paraffinic or naphthenic base oil with a hydrofined light gas oil having a sulfur to nitrogen weight ratio of greater than 100:1 wherein the hydrofined light gas oil is added to the base oil in an amount sufficient to provide a blend having greater than about 0.03 wt % sulfur.
There is a need for a mineral insulating oil that provides, for example, low temperature performance, retains good gassing tendency, and exhibits oxidation stability.
There is also a need for a mineral insulating oil that meets the requirements of various standards, for example, “Fluids for Electrotechnical Applications—Unused Mineral Insulating Oils for Transformers and Switchgears” (CEI IEC 60296) and the Standard Specification of Mineral Insulating Oil Used in Electrical Apparatus (ASTM D3487 Type I).